The invention relates to the use of ethylene-co-vinyl acetate (EVA) for the manufacture of outer soles of shoes. The vinyl acetate content of the polymer material used is 14 to 28% and materials of this type are distinguished by especially high flexibility, low weight, good wear behavior and ability to be dyed.
In German Offenlegungsschrift No. 16 85 383, a method for the manufacture of a profiled sole of elastomer with a pore structure is described, in which a quantity of the elastomer mixture corresponding to the size of the sole is placed in the sole mold which during this process is larger than the volume of the finished sole. The elastomer mixture is subsequently distributed uniformly in the hollow mold by raising a bottom plunger and is foamed to the final dimension during the subsequent slow lowering of the bottom plunger. The finished sole can then be taken from the hollow mold and is characterized by the finely developed uniform pore structure. The mentioned method nevertheless has found no acceptance because the sequence of the individual operations is time-consuming and precludes mass production at low cost. Corresponding use with repect to the manufacture of a profiled outer sole of EVA is not possible for this reason.
In Austrian Patent Application No. 67 57/78, a method for the manufacture of a molded shoe part with a relief-like structured surface of a cross-linked polyolefin foamed with closed cells is described. For this purpose a foam material blank with specific dimensions is first formed from the material used and then formed in the desired manner into a mold through the use of a combined heat-pressure treatment. This results in heavy densification of the foam material blank over the entire cross section, and as a consequence, in an increase of the specific gravity and an impairment of the flexibility. The pore size and distribution vary extremely as a function of the degree of densification, which is detrimental with respect to the wear behavior. For this reason, an outer sole cannot be made in this manner. The processing of EVA is not mentioned in the cited literature reference.
It is, therefore, an object of the invention to develop an efficient method for the mass production of a profiled outer sole of a cross-linked EVA foamed with closed cells, which ensures the achievement of a homogeneous and uniformly fine pore structure. Such soles are commonly known as waffle soles, "Vibram".RTM. soles and other similar types with lugs, ridges or other gripping features on the bottom of the sole. It is desired thereby to achieve the manufacture of an outer sole which is distinguished by high flexibility with low weight and good wear behavior.